The method is for treating a liquid or slurry with an ultrasonic energy. A movable endless member is provided that is permeable to a liquid. A transverse foil is disposed below the member and extends across the member and a transducer is in operative engagement with the foil. The member is moved about rollers. The transducer generates pressure pulses into the foil to form implosion bubbles in the liquid disposed above the member. The implosion bubbles have a first diameter (d1). A gap is formed between the member and the foil. The gap represents a distance (d2) that is less than the diameter (d1) of the bubbles to prevent any bubbles of critical size to be captured in the gap.